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Help With JCM 2000 Hum!!!

Discussion in 'The Workbench' started by scoobyjr, Dec 19, 2020.

  1. scoobyjr

    scoobyjr Member

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    Hey everyone - I have a 2003 Marshall JCM 2000 DSL 100 head. All original - even original tubes. The amp has never given me any problems and prior to March 2020, I used it with the band regularly. Recently, the amp has developed a low "hum" when I turn it on and it is on standby. It gets much louder when it take it off standby. The hum is the same whether there is a cable plugged into the input or not. No matter what guitar I use or what speaker cable or speaker cabinet I use, no change to the hum. To see if the problem was from lights, I unplugged all the lights and there in the dark the hum was the same. No change to the hum if I press any of the buttons on the amp or change channels. I have tried turning each knob to zero individually and that does not change the hum at all. Is there anything I can do or try before spending money to take the amp to a tech??? Thanks!
     
  2. The Dose of Harmony

    The Dose of Harmony Well-Known Member Gold Supporting Member

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    What year is it?

    Tubes maybe!
    When was last time you retube?
    Any spare tubes around?
    Does it do it with the guitar plugged
     
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  3. scoobyjr

    scoobyjr Member

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    BOTH PLUGGED AND UNPLUGGED
     
  4. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Yep agreed. Try tubes, first power I would say esp because tubes are original. You have to buy a set of 4 and before you worry about biasing etc change em and see.
     
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  5. scoobyjr

    scoobyjr Member

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    okay, power tubes first. Cool. Any recommendations?
     
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  6. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Whatever is handy that you can get. Everyone likes something different! If you have a local store that would be easy, onljne opens up to you name it from everywhere!
    I should add - many amps have a switchjng input jack so jack is shorted when there is nothing plugged in. Because you still get hum that means it is jnternal, the amp is creating the hum. That is a good long time for power tubes to last!
    Try JJs, EH, whatever
     
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  7. The Dose of Harmony

    The Dose of Harmony Well-Known Member Gold Supporting Member

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    Well thats a whole journey right there!
    If you never have retube i would change all them pre amp and power tubes

    i dont have a Dsl
    But on my amps i like mullard ,tungsol , jj for power tubes!
    Pre amp i always like to buy different brands
     
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  8. South Park

    South Park Well-Known Member

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    A hum could be from the heaters on one of the tubes . Start with the first preamp tube by pulling to locate the hum then the rest of them
     
  9. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Could be preamp too. My start would be power given the age. OP needs a spare set for sure. I would get them and then try them. Failing that try pre tubes one at a time.
    Could try that first too. Preamp tubes tend to last eons but it could narrow it down before getting power tubes.
     
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  10. scoobyjr

    scoobyjr Member

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    Update - I put the amp on standby. I plugged in my guitar and placed it on the stand. Same hum. I have an extra 12AX7, so I pulled out V1 - still hum. I put in extra tube in V1. Same hum. I set tubes back to normal and pulled V2. Still humming. I put extra tube in V2. No difference. At that point I stopped and turned the amp off.
     
  11. The Dose of Harmony

    The Dose of Harmony Well-Known Member Gold Supporting Member

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    Just get a whole new set!
    If not tubes might need a recap maybe!
     
  12. The Dose of Harmony

    The Dose of Harmony Well-Known Member Gold Supporting Member

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    You might check out this video!
    Pretty good for troubleshooting!
     
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  13. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    Pre amps are good. Chances are very good, IMO, that you are starting to have a failing power tube especially when they are 17 years old.

    Could need a recap but it isn't very old for that. Start with the basics!
     
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  14. Dogs of Doom

    Dogs of Doom >>> Moderator <<< Staff Member

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    you could try biasing them, but, I'd be getting new tubes soon.

    When one side drops below a certain point, the balance gets off of the push/pull system. If you can bias the tubes within range of each other, you might get rid of the hum...
     
  15. scoobyjr

    scoobyjr Member

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    Thanks everyone! Ok, I pulled out the multimeter and now I just need to order some tubes!
     
  16. Jethro Rocker

    Jethro Rocker Well-Known Member Silver Supporting Member

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    They can do all kinds of crazy stuff. I have only had a few failures over the years. Had amp smell like hot burnjng amd it took out the HT fuse.
    Had an amp just suddenly stop.
    I have had static when first turned on which faded after awhile. I was sure it was a pre tube but nope. Power tubes.
     
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  17. Dogs of Doom

    Dogs of Doom >>> Moderator <<< Staff Member

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    just for grins. check the bias & report back. Try to bias.
     
  18. What?

    What? Well-Known Member

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    I wouldn't run it this way for regular playing, since I have read and heard about too many issues with modern Marshall output transformers being fragile and biting the dust, but you could pull two power tubes at a time (one from each side, not two from one side) and figure out if one of them is humming. Don't pull them with the amp running of course, and it would be a good idea to rebias for 2 tubes and to address the impedance issue of running only two power tubes. In other words, if you have a 16 ohm cab connected to the 16 ohm tap, connect it to the 8 ohm tap while only 2 power tubes are in there.

    Or you can just throw money at new tubes, maybe find the problem, maybe not. But it never hurts to have a set of backup tubes anyway.
     
    Last edited: Dec 19, 2020
  19. South Park

    South Park Well-Known Member

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    Rolling some tubes is just the first step in trouble shooting . Try to wiggle the tubes and if things change it might be bad connection in the tube socket
     
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  20. titan7

    titan7 Active Member

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    Both my DSL 50s hum have since the day I took them out of the box.
     
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